Conventionally, an engine generally employs a mechanical water pump for pressure-feeding or circulating coolant. The mechanical water pump is driven by the output of the engine, and the flow rate (discharging volume) depends on the rotational number of the engine. In contrast, there is known a variable water pump (for example, an electric water pump) applicable to the engine and capable of changing the flow rate of the coolant pressure-fed in order to improve the engine warm-up characteristics.
As for such a variable water pump, for example, Patent Document 1 describes a technique which causes the coolant to intermittently flow when the coolant temperature is equal to or lower than the value beforehand set. Also, for example, Patent Document 2 describes a technique which stops the electric water pump when the coolant temperature is lower than a predetermined value. Also the technique intermittently drives the electric water pump at predetermined intervals when the coolant temperature is equal to or higher than the predetermined value.
Also, for example, Patent Document 3 describes a technique which drives the electric water pump for a predetermined period when the engine starts. Also, this technique stops the electric water pump when the coolant temperature during the driving of the electric water pump is equal to or lower than a predetermined value. In the related technique described in Patent Document 3, the stop control of the electric water pump is finished based on at least one of the coolant temperature, an accumulated intake air quantity during the stopping of the electric water pump, and a stop period thereof. Further, Patent Document 3 describes a technique which drives the electric water pump for a predetermined period when a stop period of the electric water pump is longer than a predetermined period. Also, the technique continuously stops the electric water pump when the coolant temperature during the driving of the electric water pump is equal to or lower than a predetermined value.
Patent Document 1: Japanese Patent Application Publication No. 2006-214281
Patent Document 2: Japanese Patent Application Publication No. 2004-316472
Patent Document 3: Japanese Patent Application Publication No. 2008-169750